The present invention relates to waveform correction or control using inductance-controllable inductors and, more particularly, to the control or correction of waveforms used to drive raster-type CRT displays and, still more particularly, to the control or correction of waveforms used to drive the horizontal sweep of raster-type displays and monitors such as those used in televisions and computer displays.
Various systems have been developed to control the electron beam in cathode-ray tubes (CRT's). The classic system utilizes a set of coils, i.e., the deflection yoke, mounted on the neck of the tube to control both the horizontal and vertical movement of the electron beam. In general, horizontal control is effected by successively deflecting the electron beam from the left to the right at a selected frequency with the vertical position of the beam synchronously controlled with successive horizontal sweeps. The circuits that effect horizontal control, known generically as the horizontal sweep circuits in the television industry and as the horizontal scan circuits in the computer monitor industry, generate a recurring waveform (typically having a sawtooth or sawtooth-like form) that, in turn, drives the deflection coils to sweep the electron beam from the left to the right. Because of frequency-dependent power losses in the deflection yoke, non-linearities in the transfer characteristics of the output power transistors, and other error sources in the horizontal drive electronics (including capacitive and inductive effects), certain non-linearities are introduced into the system that cause distortion in the displayed image. The distortion can take the form, for example, of a displayed image that is wider at the top and narrower at the bottom, pincushion effects by which the center portion of the display appears narrower than the upper or lower portions, and/or foldback. As displays have become larger (both in the case of television receivers and computer displays) and as horizontal sweep frequencies have become higher (in computer monitors), these non-linearities become more prominent and increase the need for correction of the resulting distortion/non-linearity errors in the displayed image.
Over the decades, various types of circuits and devices have been developed to compensate for the intrinsic non-linearities. These prior art circuits have included switched-inductor devices that vary the inductance of an LC circuit at selected portions in the waveform to correct for distortion in the displayed image. Historically, television displays and computer displays have operated at a single horizontal scanning frequency; accordingly, the design parameters for effecting waveform compensation or correction have been relatively straightforward. In recent years, the trend in the design of computer monitors has been in the direction of higher resolution displays that can effect horizontal scanning at one of several possible scanning frequencies, including frequencies in the area of 70-80 KHz. This trend has exacerbated the problem of waveform correction or compensation since a wider range of inductance is required in the waveform correction circuitry.